I just came across a new website that promises online screen writing functionality. It promises to make collaboration easier. I haven't used it myself, but it looks like it might be something I try in the future.

From the about page:

The advantages of web-based applications are obvious - no software to install, no updates to keep track of, easy portability, and automatic off-site backup of data. Screenwriting, in particular, enjoys significant benefits from this architecture. By hosting the software online, we are able to offer easy and intuitive collaborative features. Users can work privately as they do today, collaborate with a few friends, or even collaborate with the entire web community. We automatically manage revision history and maintain a record of authorship. Also, by offering a web-based application, we can easily reach a diverse population of writers and build a dynamic community that is not possible with traditional screenwriting applications.